Dissolved nitrous oxide and hydroxylamine in the northern South China Sea in summer

Seawater samples from the nrorthern South China Sea were collected aboard the R/V Zhong Shan Da Xue during June 22 - July 10, 2022. Discrete N2O samples were drawn from 10 L Niskin bottles mounted on a CTD rosette system. 30 mL glass vials were used to collect water samples after overflowing. All samples were free of air bubbles and sealed with rubber stoppers and aluminum caps. Saturated mercuric chloride (HgCl2) solution was injected into each vial immediately to inhibit microbial activity. Triplicate samples were collected and stored in dark until lab analysis after the cruise. N2O concentration was measured with a static headspace-gas chromatography method according to Walter et al. (2006).Triplicate bubble‐free samples were collected from the Niskin bottle. NH2OH in seawater was measured with an improved ferric ion oxidation method as described in Kock and Bange (2013). N2O concentrations were calculated based on the empirical formulas by Weiss and Price (1980). The value of excess N2O (ΔN2O) was calculated by the difference between observed N2O concentration [N2O]ob and the atmospheric equilibrium concentration [N2O]eq. [N2O]eq can be calculated as a function of temperature, salinity and the volume fraction of N2O in the atmosphere. The atmospheric N2O mixing ratio can be found on National Oceanic and Atmospheric Administration/Global Monitoring Laboratory measurements. The air-sea flux of N2O (F) is a function of gas exchange coefficient k and ΔN2O in surface seawater. Dissolved oxygen (DO) concentrations were measured with a sensor attached to the CTD-Rosette with an accuracy of 0.5%. A Winkler titration method was adopted on board to calibrate the O2 data. Nutrient samples were frozen after filtration on board. NH4+, NO2-, NO3- and PO43- were measured with the classic colorimetric methods with an auto-analyzer (QuAAtro and AutoAnalyzer 500, Germany). N* was defined as the excess of total inorganic nitrogen (DIN) compared to phosphate.Spearman correlation was used to study the monotonicity relationship between environmental factors and N2O and NH2OH. Structural equation modeling (SEM) was used to analyze contribution of these factors to N2O and NH2OH variability (Fox et al., 2012). We adopted R package lavaan for modeling and adheres strictly to the modeling criterion (Rosseel, 2012).Data from the National Oceanic and Atmospheric Administration/Global Monitoring Laboratory measurements, available at https://gml.noaa.gov/ccgg/trends_n2o/, were used to calculate excess N2O. The software R version 4.2.3 associated with this manuscript for the structural equation model is licensed under MIT and published on R-hub https://r-hub.github.io/rhub/ (Csárdi & Salmon, 2023). Figures were made with Origin version 2022 (OriginLab Corporation, Northampton, MA, USA). Maps and parts of the figures were created through Ocean Data View version 5.6.3 (Schliter, Reiner, http://adv.awi.de, 2021).